Total hip replacement is one of the most commonly performed orthopedic procedures. In such a procedure, the acetabulum of the hip is replaced with a cup-shaped socket device which receives and articulates with a prosthetic femoral implant which consists of a ball portion and a stem portion which inserted fixedly in the femur. The femur is the longest bone in the skeleton, and is almost perfectly cylindrical in the greater part of its extent. At its proximate portion are the head, the neck, a greater trochanter and a lesser trochanter. The head is globular and generally hemispheric in shape. The surface of the femur head is smoothly contoured and, under normal conditions, moves freely within the acetabulum of the hip.
The neck of the femur is a pyramidal process of bone connecting the head with the femur body, disposed at approximately a 135.degree. angle relative to the shaft of the bone. The greater trochanter is a large, irregular, quadrilateral eminence which projects from the angle of the junction between the neck of the femur and the body. The lesser trochanter is a conical imminence which projects posteriorly from the base of the neck. An intertrochanteric line runs obliquely from the greater trochanter to the lesser trochanter on the anterior surface of the femur.
Because of the affects of age, injury and/or disease, patients all too frequently require hip replacement. In such a surgical procedure, the prosthetic cup-shaped device, generally composed of a high molecular weight polyethylene (sometimes supplemented by a metallic shell), is fixed into the acetabulum with use of bone cement or by other mechanical means, and the replacement of portions of the femur with the usually metallic "ball and stem" device, stem portion of which fixed into the femur with either cement or other mechanical means. The prosthetic devices cooperate swivelly to permit articulation between the femur and the hip. As a result, the patient gains mobility of the lower extremity and freedom from pain. During surgery, the proximal portion of the femur comprising the head and neck, are removed, generally by incision above the intertrochanteric line.
In general, the femoral prosthetic device is also composed of corrosion resistant metal alloy. The femoral device includes a head, a neck, and a stem. The stem is adapted for insertion rigidly and securely within an opening formed in the femur, after the femoral head and neck have been surgically removed. The proximal femur os filled with bone cement, then the femoral implant is inserted.
After the acrylic cement solidifies it acts as a filler to firmly secure the implant to the femur. After patient has recovered from the surgical procedure, in successful cases, the patient is able to move about with more freedom and less pain than previously. With such an increasing number of hip replacement surgeries being performed, there is a concomitant increase in failure of the implant. This failure may be caused by infection. However, it is frequently caused by a mechanical loosening between the osseous tissue and implant. Fixation failure is a very serious condition which, in most cases, necessitates early surgical intervention.
Whatever the cause of implant failure, a common characteristic is noted. It is loosening either at the tissue/cement interface, or at the cement/prosthetic stem interface.
When fixation at either interface fails, a serious condition exists, because of the occurrence of movement of the stem in relation to the bone. In such cases, the stem acts as a rasp to abrade the bone tissue. Debris causes inflammation reaction which in turn causes more bone reabsorption. If not diagnosed and corrected early, such abrasion and inflammation can result in such a large amount of bone loss, that subsequent implantation may be impossible, or at least difficult to achieve. In addition to the problem of bone loss experienced in implant failure, the condition, if not corrected, results in great pain for the patient.
Therefore, in order to prevent bone loss due to implant loosening and to alleviate pain caused by a loosened implant, it would be highly desirable to have a method of discovering prosthetic implant fixation failure at an early stage. However, from the diagnostic aspect, it should be noted that pain experienced by the patient is not always a reliable indicator of implant failure, because the pain may be caused by other factors. Thus, a need exists for a reliable method for discovering fixation failure, which method does not depend upon the occurrence of pain in diagnosis.
Generally, two procedures have been utilized in the attempt to diagnose prosthesis implant integrity. One of such procedures is to inspect the bone/implant area by x-ray examination. This technique has several limitations. In the first place, the technique is not entirely accurate, since it frequently fails to identify areas of implant loosening. Additionally, the x-ray technique entails the exposure of the patient to unwanted radiation. In addition, such techniques are generally ineffective in diagnosing implant failure at an early stage. Further, the x-ray use in such a technique presents a health hazard to the patient.
Another technique for attempting to evaluate bone implant fixation integrity utilizes conventional or digital subtraction arthrography methods. These methods are limited, because they utilize a contrast medium which must be introduced in close proximity to the area of cement failure, otherwise the medium does not effectively disclose the failure.
In addition, the arthrographic techniques often produce false negatives. Reliance on such false negatives can lead to injury to the patient. Another important limitation of arthrographic techniques is the fact that it is invasive, causes patient discomfort and, has a risk of introducing infection.
Therefore, it would be highly desirable to have a method and apparatus for determining implant fixation integrity, without the necessity of employing harmful radiation. Also, such techniques should be non invasive, painless and substantially risk free.
Chung et al., (1979) reported a limited anatomical study of the alteration of natural resonant frequency of the femur during curing of the cement. They alluded to the possibility of diagnosing prosthetic loosening by small alterations in the natural resonant frequency of the bone-implant unit in secure and loose prostheses. Van der Perre (1984) reported that in the case of a loose prosthesis, there was a decrease in the resonant frequencies, and that the frequency spectrum was `rather noisy`. He also proposed that monitoring of the resonant frequencies over time would be the best method of determining loosening of the prosthesis. In previous studies, a considerable variation in the natural resonant frequency of whole cadaver femurs and in femurs with secure prosthetic implants in situ has been observed consistent with the individual differences in the mass and shape of the bones. Hence, if the phenomenon of frequency shift on loosening of the prosthesis is to effectively used, as proposed by Chung and his coleagues and by Van der Perre, prior knowledge of the resonant frequency of the bone-implant unit before loosening has ocurred is required for every patient. This information would not realistically be available in the clinical situation.
A further limitation of the Chung technique is that it is invasive, utilizing a needle driven by a speaker diaphragm. In this regard, in clinical practice, measurements could only be made by using a hypodermic needle brought into contact with the underlying bone. Such a technique would certainly be highly painful and carry the risk of infection.
Therefore, in light of the limitations in the prior known techniques, it would be highly desirable to have a method and apparatus, which would be useful in the dynamic analysis of bone implant composite integrity, in a painless and non-evasive manner. Such a technique would provide reliable clinical data, irrespective of age, bone differences. Further, it would be highly desirable if such a method and apparatus could be developed in such a manner that they could be utilized at any time after prosthetic implantation, without the requirement for acquisition of base-line reference values immediately after implantation surgery.